U.K Patent 2,243,264B issued to Cintel discloses the use of a modified telecine machine to scan at high definition. The film is scanned in "Non Real Time", with each frame being scanned a number of times. Each partial scan for a frame covers only a portion of the pixels. The partial scans are then re-combined to produce data representing the entire frame, and output can then be provided at true film rate. In Cintel's Patent, each frame is considered as divided into a number of pixels suitable for high definition output. Each pixel will be one quarter of the size of normal definition pixels relative to the film and there will be four times as many of them per frame. The frame is then divided into a number of blocks, each containing four of the high definition pixels. A first scan is carried out, in which only one pixel in each block is analysed. The data is stored. The scan is then shifted and another pixel in each block is then analysed. This is repeated until each pixel in each block has been analysed. In practice therefore using a block consisting of two pixels on one scanning line and two on the next scanning line, there will be four interleaved partial scans, one covering odd lines and odd pixels, one odd lines and even pixels, one even lines and odd pixels and finally even lines and even pixels. The information from the four partial scans is then recombined to provide data representing the entire frame, and this data is then stored in a frame store for subsequent output. In this manner, a telecine machine which has been designed essentially for normal definition output can be modified to produce high definition output.
In my co-pending U.S. patent application Ser. No. 08/317,329 entitled "High Definition Colour Correction" filed on Oct. 4, 1994 I disclose a system for colour correcting images in such high definition systems. In addition to the possibility of interleaved scanning as disclosed in Cintel. I also refer to the possibility of dividing the frame into four quadrants which are scanned separately, such as top left, top right, bottom left and bottom right.
In the above systems, it is assumed that there is no problem in re-constituting the image of the entire frame from the partial scans. For example, in Cintel the system knows that the successive partial scans have been shifted by one pixel horizontally and/or vertically and it is this displacement that will be assumed when the complete image is re-constituted. However, I have now realised that there can be inaccuracies in this assumption. The nature of the above systems is that each partial scan takes the same time as a complete scan at normal resolution. To completely scan a frame with the four successive partial scans will take four times as long as scanning a frame once at normal resolution. In this extended period, errors may occur which will affect undesirably the spatial alignment of the successive partial scans.
In a flying spot system, for example, one source of errors will be possible fluctuations in the background magnetic field between the successive partial scans. Such changes in background magnetic field will affect the deflection of the electron beam with the cathode ray tube and thus the position of the light spot formed on the phosphorised face of the tube. The resultant deviation of the beam of light passing to the film frame for one partial scan as compared to another partial scan will mean that positional and rotational discrepancies and discontinuities may occur when the partial scans are combined to re-constitute the complete frame. This is extremely undesirable.
Re-constituting partial images has been proposed in another field. In U.S. Pat. No. 5,386,228 dated Jan. 31, 1995, issued to Tadashi Okino, there is disclosed an image pickup device used, for example, in a home video camera. Okino addresses the problem of physically fitting together image pickup elements such as CCD's in sufficient density for higher resolution use. In Okino, this particular physical problem is solved by scanning the complete image in one pass, but then splitting the transmitted light into portions which are directed simultaneously to different arrays of pickup elements. Each of these arrays is arranged physically only to receive light from a particular part of the image. In one preferred arrangement in Okino, the light from a scan is passed simultaneously to four separate arrays, each arranged physically to respond to light form a different quadrant of the image. Because there is a single scan and the light for the four quadrants is analysed simultaneously, there is no problem with errors occurring between partial scans and Okino does not consider the manner in which the image portions from the four arrays are re-combined in spatial alignment. Okino deals with the problem that the differences in optical paths and pickup sensitivities for the four different quadrants may lead to undesirable differences in intensities, and deals with this by overlapping the quadrants and analysing the intensities in the overlapping regions. Such a problem does not arise in the telecine systems described earlier, because the partial scans are carried out sequentially and the light in each follows the same optical path to the same sensors.
The simultaneous handling of quadrants as in Okino is inappropriate for the telecine systems described earlier. Not only would the additional hardware make the apparatus extremely complicated, but there would be the problem of having to scan at high resolution rates and to handle a real time data stream comprising simultaneous data for the entire image at high resolution.
There thus remains as a need for a telecine system in which there can be carried out successive partial scans at normal resolution, followed by re-constitution of the entire image at high resolution, but without the problems arising from discrepancies arising during the period over which the successive scans are carried out.